The use of permanent magnet DC motors has become widespread, due at least in part to its lower cost as compared to other types of motors. Electronic circuits have been devised to control the direction of current in the motor armature. By reversing the direction of current flowing through the motor armature, the rotational direction of the motor can be controlled. so as to operate the motor in two directions, i.e. clockwise rotation and counter-clockwise rotation. Examples of existing motor reversing circuits are disclosed in U.S. Pat. Nos. 3,229,181, issued to A. C. Evans; 3,305,718, issued to C. F. Waldron, 4,987,353, issued to Imaeda; 5,023,493, issued to Wrzesinski; and 5,132,600, issued to Kinoshita.
Referring now to FIG. 1, there is illustrated an existing circuit 10 for reversing the rotational direction of a motor 16. As shown, the circuit 10 includes a control switch 22 and relays 24 and 28 which are electrically interconnected. The motor brushes 12 and 14 are electrically connected to negative voltage through normally closed relay contacts shown generally by reference numerals 18 and 20, respectively. When the control switch 22 is moved to the clockwise (CW) position, the coil associated with the relay 24 is energized, actuating the relay contacts 18 and 26. As a result, positive voltage is applied to the motor brush 12 through the normally open contact 26 associated with the relay 24. With the brush 14 of the motor still connected to negative voltage through the normally closed contact 20 associated with relay 28, the motor 16 is operated in a clockwise direction.
With continuing reference to FIG. 1, if the control switch 22 is moved to the counterclockwise (CCW) position, the coil associated with the relay 28 is energized, actuating the relay contacts 20 and 30. As a result, positive voltage is applied to the motor brush 14 through the normally open contact 30. With the motor brush 12 still connected to negative voltage through the normally closed contact 18, the motor runs in a counterclockwise direction.
A primary disadvantage associated with existing reversing circuits, including the circuit shown in FIG. 1, is the fact that when the control switch 22 is switched from either the CW or CCW position to the "OFF" position, the motor 16 is electrically shorted. For example, if the motor 16 is running in counter-clockwise direction, current is flowing through the motor from brush 14 to brush 12. When the control switch is moved to the "OFF" position, the motor armature continues to rotate, generating a large current which flows in the same direction from the motor 16 through the brush 12 and the normally closed contact 18. The normally closed contact 18 and the normally closed contact 20, being electrically connected to the same point, create a closed circuit path for this current to follow. Thus, the generated current also flows through the normally closed contact 20 and back to the motor 16 through the brush 14. This large current has been shown to significantly shorten the life and degrade performance of the motor brushes and the relay contacts over time.